The present invention relates to an ultrasonic imaging system for visualizing the structure and movement in a living body.
FIG. 1 shows a construction of a conventional ultrasonic imaging system. In FIG. 1, an ultrasonic probe 51 converts a pulse signal into an ultrasonic signal, transmits the converted ultrasonic signal to an object under measurement, receives an echo signal from the object under measurement, and converts the echo signal into an electrical signal. Driver means 52 generates the pulse signal and supplies it to the ultrasonic probe 51. An amplifier 53 amplifies the echo signal returning from the object under measurement and received by the ultrasonic probe 51. Logarithmic conversion (compression) means 54 performs logarithmic conversion of the amplitude of the echo signal amplified by the amplifier 53. Detector means 55 performs AM detection of the logarithmically compressed echo signal and detects an envelope of the echo signal. Scanning conversion means 56 composed of an A/D converter 56a, a memory 56b and a D/A converter 56c, takes in a detection signal of the echo signal, which has been AM detected by the detector means 55, and converts the detection signal to an image signal according to a TV system represented by the NTSC system to form a two-dimensional tomographic image. Display means 57 such as a television displays a tomographic image indicating inner portions of the object under measurement. Generally, a low-pass filter for preventing generation of an aliasing noise on the basis of the sampling theorem is provided at an input portion of the A/D converter 56a. However, this low-pass filter is different from filter means of the present invention, and it is not illustrated here in order to simplify the explanation of the present invention.
With respect to the above-described construction, the operation thereof will be described hereunder.
The ultrasonic probe 51 is driven by a pulse signal generated by the driver means 52 to perform transmitting and receiving operations by transmitting an ultrasonic pulse, while performing a scanning operation with the transmitted ultrasonic pulse, and by receiving an echo signal returning from a tomographic plane in an object under measurement. The echo signal thus obtained is amplified by the amplifier 53 and its amplitude is subjected to logarithmic conversion by the logarithmic conversion (compression) means 54. The logarithmically converted echo signal is subjected to AM detection by the detector means 55 so that the detection signal represents an envelope of the echo signal, and the detection signal is outputted to the scanning conversion means 56. The detection signal is converted to a digital signal by the A/D converter 56a of the scanning conversion means 56, and the A/D conversion signal is written in the memory 56b. Since an address of the memory 56b, in which the A/D conversion signal data is written, is made to correspond to a position of the portion of the object under measurement from which the echo signal has been generated, tomographic image information of the object under measurement is stored in the memory 56b, being arranged two-dimensionally therein. The tomographic image information stored in the memory 56b is subjected to D/A conversion by the D/A converter 56c so as to comply with the television display system for the display means 57 and is outputted to the display means 57, whereby a tomographic image is displayed on the display means 57.
As described above, in the conventional ultrasonic imaging system, the ultrasonic probe 51 performs transmitting and receiving operations of an ultrasonic pulse, while performing a scanning operation, to thereby obtain a tomographic image of portions of an object under measurement.
In the conventional ultrasonic imaging system, however, an image displayed on a television display used as the display means 57 is represented by the scanning line. Accordingly, the resolution of an image which can be displayed is determined by the television scanning line. In addition, even if the resolution of an image is retained within a resolution limit, when displaying a tomographic image of a fine composition which is near the resolution limit, a difference in brightness between adjacent television scanning lines is increased due to a high spatial frequency of the tomographic image. As a result, it becomes impossible to represent intermediate brightness, which raises a problem that the quality of a diagnostic image is deteriorated significantly.